Feel free to give me your opinion, but I think an Intake
(either cold air or Short Ram
) is just a pipe
and nothing more. That does not require any huge amount of engineering. I think it's fine to make one out of PVC for yourself, too. It may be true about the harmonic frequencies of the tube are taken into account so that standing waves form. However, keep in mind that an engine runs at many different frequenices. Even so, the signifigance of this to the power gain would be negligable, with the main effect being non restricted air flow. I would imagine that they could sound differnt at different rpms.
I not an expert though. Pros to a brand name would be quality and the coolness factor. I personaly think it's pretty cool to make your own stuff though.
PS. eBay is good. I've spent and made thousands of dollars on eBay with no problem ever.